feat: add LoadingScreen with full-screen triangle shader

Assets/shaders/loading.frag

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+#version 330 core
+out vec4 FragColor;
+void main() { FragColor = vec4(0.10, 0.12, 0.16, 1.0); }

Assets/shaders/loading.vert

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+#version 330 core
+layout(location = 0) in vec2 aPos;
+void main() { gl_Position = vec4(aPos, 0.0, 1.0); }